312 



Mr. W. Crookes on the Radiometer. 



[Nov. 16, 



heat, either by grasping the bulb with the warm hand, dipping it into hot 

 water, or covering it with a hot glass shade, it rapidly rotates in a nega- 

 tive direction, the black advancing, and continuing to do so until the tem- 

 perature has become uniform throughout. On now removing the source 

 of heat, the fly commences to revolve with rapidity the positive way, 

 the black this time retreating as it would if light shone on it. Pith or mica 

 radiometers act differently to this, dark heat causing them to revolve in 

 the same direction as light does. 



The outer corners of the aluminium plates, which were mounted dia- 

 mond-wise, were now turned up at an angle of 45°, the lampblacked surface 

 being concave and the bright convex. On being exposed to a candle, 

 scarcely any movement was produced ; when one vane was shaded off the 

 other was repelled slightly, but the turned-up corner seemed to have almost 

 entirely neutralized the action of the black surface. A greater amount of the 

 same corner was now turned up, the fold going through the centres of ad- 

 jacent sides. Decided rotation was now produced by a candle, but the black 

 surface was attracted * instead of repelled. Dark heat still caused the 

 opposite rotation to light, repelling the black surface. 



The plates were now folded across the vertical diagonal, the black sur- 

 face being still inside and the bright metal outside. The actions with a 

 candle and hot glass shade were similar to the last, but more decided. 



The plates were now flattened, and put on the arms at an angle, still 

 being in the vertical plane. When the bright surface was outside, scarcely 

 any action was produced by a candle ; but when the lampblacked surface 

 was outside, strong repulsion of the black was produced, both with a candle 

 and with a hot shade. 



The square aluminium plates were mounted in the experimental appa- 

 ratus, one being attached to the arm by the centre of one of the sides, and 

 the other by an angle. The opposite corner of the one mounted diamond- 

 wise was turned up at an angle. The outer convex surface of the diamond 

 plate was blacked, and the side of the square plate facing the same way 

 was also blacked, so that either two black or two bright surfaces were 

 always exposed to the light, instead of a black and a white surface, as is 

 usual in radiometers. As might have been expected, both these black sur- 

 faces were repelled ; but the turned-up corner of the diamond-mounted 

 plate proved so powerful an auxiliary to its black surface, that strong 

 rotation was kept up, the square plate being dragged round against the 

 action of light. 



Folding the plates with the angle horizontal has not so decided an action 

 as when the fold is vertical. 



Sloping the plates and disks of a lampblacked mica radiometer so as to 



* I use the word attraction in these cases for convenience of expression. I have 

 no doubt that what looks like attraction in these and other cases is really due to vis a 

 tergo. 



